Better living through reliability.
As part of our SRE team's postmortem practices, we started having quick "pre" meetings before the full postmortem meetings. We'd do an abbreviated version of a regular postmortem meeting, but only with the SRE team. We communicated this practice to the team and added it to postmortem meeting checklists. But new members on the team lacked context on the "why" of these meetings, which is a good question to answer and document.
Postmortem practices mature over time, so in the beginning certain aspects of the postmortem itself or the postmortem meeting might need some polish.
There had been times where we'd come into a postmortem meeting with large holes in the postmortem or without a good focus on what needed to be discussed with the larger audience and stakeholders. We wanted to ensure that when we got in the room with everyone, we were well prepared, not wasting anyone's time, and would be able to focus and drive the larger issues that need to be addressed in that meeting. Additionally we didn't want to blindside any presenter, so hopefully anything not passing muster would come out beforehand. Ideally that can be done async via postmortem document comments, but not always. The last thing we wanted was to waste everyone's time and reschedule another postmortem meeting if there are too many gaps (which is often painful to reschedule stakeholders and embarrassing for the presenter). A quick meeting beforehand is designed to prevent all of that.